Physicalism

Physicalism is the thesis that everything is physical, or as
contemporary philosophers sometimes put it, that everything supervenes
on the physical. The thesis is usually
intended as a metaphysical thesis, parallel to the thesis attributed
to the ancient Greek philosopher Thales, that everything is water, or
the idealism of the 18th Century philosopher Berkeley, that everything
is mental. The general idea is that the nature of the actual world
(i.e. the universe and everything in it) conforms to a certain
condition, the condition of being physical. Of course, physicalists
don't deny that the world might contain many items that at first
glance don't seem physical — items of a biological, or
psychological, or moral, or social nature. But they insist
nevertheless that at the end of the day such items are either physical
or supervene on the physical.

Physicalism is sometimes known as ‘materialism’.
Indeed, on one strand to contemporary usage, the terms
‘physicalism’ and ‘materialism’ are
interchangeable. But the two terms have very different histories.
The word ‘materialism’ is very old, but the word
‘physicalism’ was introduced into philosophy only in the
1930s by Otto Neurath (1931) and Rudolf Carnap (1959/1932), both of
whom were key members of the Vienna Circle, a group of philosophers,
scientists and mathematicians active in Vienna prior to World War II.
It is not clear that Neurath and Carnap understood physicalism in the
same way, but one thesis often attributed to them (e.g. in Hempel
1949) is the linguistic thesis that every statement is synonymous with
(i.e. is equivalent in meaning with) some physical statement. But
materialism as traditionally construed is not a linguistic thesis at
all; rather it is a metaphysical thesis in the sense that it tells us
about the nature of the world. At least for the positivists,
therefore, there was a clear reason for distinguishing physicalism (a
linguistic thesis) from materialism (a metaphysical thesis).
Moreover, this reason was compounded by the fact that, according to
official positivist doctrine, metaphysics is nonsense. Since the
1930s, however, the positivist philosophy that under-girded this
distinction has for the most part been rejected—for example,
physicalism is not a linguistic thesis for contemporary
philosophers—and this is one reason why the words
‘materialism’ and ‘physicalism’ are now often
interpreted as interchangeable.

Some philosophers suggest that ‘physicalism’ is
distinct from ‘materialism’ for a reason quite unrelated
to the one emphasized by Neurath and Carnap. As the name suggests,
materialists historically held that everything was matter —
where matter was conceived as “an inert, senseless substance, in
which extension, figure, and motion do actually subsist”
(Berkeley, Principles of Human Knowledge, par. 9). But physics itself
has shown that not everything is matter in this sense; for example,
forces such as gravity are physical but it is not clear that they are
material in the traditional sense (Lange 1865, Dijksterhuis 1961,
Yolton 1983). So it is tempting to use ‘physicalism’ to
distance oneself from what seems a historically important but no
longer scientifically relevant thesis of materialism, and related to
this, to emphasize a connection to physics and the physical sciences.
However, while physicalism is certainly unusual among metaphysical
doctrines in being associated with a commitment both to the sciences
and to a particular branch of science, namely physics, it is not clear
that this is a good reason for calling it ‘physicalism’
rather than ‘materialism.’ For one thing, many
contemporary physicalists do in fact use the word
‘materialism’ to describe their doctrine (e.g. Smart
1963). Moreover, while ‘physicalism’ is no doubt related
to ‘physics’ it is also related to ‘physical
object’ and this in turn is very closely connected with
‘material object’, and via that, with
‘matter.’

In this entry, I will adopt the policy of using both terms interchangeably,
though I will typically refer to the thesis we will discuss as
‘physicalism’. It is important to note, though, that
physicalism (i.e. materialism) is associated with a number of other
metaphysical and methodological doctrines. We will return to some of
these when we discuss
Physicalism and the Physicalist World-picture.

In approaching the topic of physicalism, one may distinguish what I
will call the interpretation question from the truth
question. The interpretation question asks:

What does it mean to say that everything is physical?

The truth question asks:

Is it true to say that everything is physical?

There is obviously a sense in which the second question
presupposes an answer to the first — you need to know what a statement
means before you can ask whether it's true — and we will begin with the
interpretation question. Nevertheless, the issues here turn out to be
somewhat technical, and those new to the topic might like to read only
the first section of our discussion of the interpretation question,
which is:
Supervenience Physicalism: Introductory,
and
then turn directly to the truth question which begins at
The Case Against Physicalism I: Qualia and Consciousness.

The interpretation question itself divides into two sub-questions,
which I will call the completeness question and the
condition question. The completeness question asks:

What does it mean to say that everything is physical?

In other words, the completeness question holds fixed the issue of what
it means for something to satisfy the condition of being physical, and
asks instead what it means for everything to satisfy that
condition. Notice that a parallel question could be asked of Thales:
assuming we know what condition you have to satisfy to be water, what
does it mean to say that everything satisfies that condition?

The condition question asks:

What does it mean to say that everything is physical?

In other words, the condition question holds fixed the issue of what it
means for everything to satisfy some condition or other, and asks
instead what is the condition, being physical, that everything
satisfies. Notice again that a parallel question could be asked of
Thales: assuming we know what it is for everything to satisfy some
condition or other, what is the condition, being water, that according
to Thales, everything satisfies? In discussing the interpretation
question, I will turn first to the completeness question, and then
consider the condition question.

One answer to the completeness question – an answer suggested
initially in Davidson 1970 – looks to the notion of supervenience.
(This notion is historically associated with meta-ethics, but it has
received extensive discussion in the general metaphysics and logic
literature. For a survey, see
supervenience.)
As we shall see as we proceed, supervenience is not quite as popular
an answer to the completeness question as it once was, but it is
nevertheless remains an attractive answer, and we may go a
considerable distance by concentrating on it.

The idea of supervenience might be introduced via an example due to
David Lewis of a dot-matrix picture:

A dot-matrix picture has global properties — it is
symmetrical, it is cluttered, and whatnot — and yet all there is to
the picture is dots and non-dots at each point of the matrix. The
global properties are nothing but patterns in the dots. They supervene:
no two pictures could differ in their global properties without
differing, somewhere, in whether there is or there isn't a dot (1986,
p. 14).

Lewis's example gives us one way to introduce the basic idea of
physicalism. The basic idea is that the physical features of the world
are like the dots in the picture, and the psychological or biological
or social features of the world are like the global properties of the
picture. Just as the global features of the picture are nothing but a
pattern in the dots, so too the psychological, the biological and the
social features of the world are nothing but a pattern in the physical
features of the world. To use the language of supervenience, just as
the global features of the picture supervene on the dots, so too
everything supervenes on the physical, if physicalism is true.

It is desirable to have a more explicit statement of physicalism, and
here too Lewis's example gives us direction. Lewis says that, in the
case of the picture, supervenience means that “no two pictures
can be identical in the arrangement of dots but different in their
global properties”. Similarly, one might say that, in the case
of physicalism, no two possible worlds can be identical in their
physical properties but differ, somewhere, in their mental, social or
biological properties. To put this slightly differently, we might say
that if physicalism is true at our world, then no other world
can be physically identical to it without being identical to it in all
respects. This suggests the following account of what physicalism is
(in the following formulation and in subsequent ones, we use
“iff” to abbreviate “if and only if”):

(1) Physicalism is true at a possible world w iff
any world which is a physical duplicate of w is a duplicate of
wsimpliciter.

If physicalism is construed along the lines suggested in (1), then we
have an answer to the completeness question. The completeness question
asks: what does it mean to say that everything is physical.
According to (1), what this means is that if physicalism is true, there
is no possible world which is identical to the actual world in every
physical respect but which is not identical to it in a biological or
social or psychological respect. It will be useful to have a name for
physicalism so defined, so let us call it supervenience
physicalism.

Supervenience physicalism is relatively simple and clear, but when
construed as a formulation of physicalism, it faces four problems.
(There is also a fifth problem to be discussed later,
viz., whether supervenience is sufficient for physicalism.)
These are: (a) the lone ammonium molecule problem;
(b) the modal status problem; (c) the epiphenomenal ectoplasm problem;
and (d) the blockers problem. As we will see, while some of these are
relatively easily answered, others are much more challenging.

(Cf. Kim 1993.) Imagine a possible world W* that is physically
exactly like our world except in one trivial respect: it has one extra
ammonium molecule located, say, on Saturn's rings. It is natural to
suppose that at W*, the distribution of mental properties is
exactly as it is in the actual world — the presence of an extra
molecule does not make that much of a difference. On the other hand,
for all (1) says, such a world might be radically different in
terms of the distribution of mental properties. Since (1) only refers
to worlds that are exact minimal physical duplicates of our world, it
is silent on worlds that are different even in minute details,
and hence is silent on W*. It thus leaves open the possibility
that, in W*, everything has mental properties, or nothing has
or the only things that have mental properties are Saturn's rings! But
that seems absurd: while W* is clearly not a physical
duplicate of our world, for it contains an extra molecule, the
distribution of mental properties in W* would nevertheless
match that of the actual world.

There are a number of different responses to this problem
in the literature (cf. Kim 1993). Perhaps the simplest response is
that the problem conflates two issues that are better kept apart: the
question of what physicalism itself tells us about W*, and the
question of what our general knowledge tells us about W*. It
is true that physicalism itself does not tell us anything about the
distribution of mental properties at W*. Nevertheless, we know
independently what the distribution is — we know independently that
the presence or absence of molecules on Saturn doesn't affect things
like who has mental properties here on Earth. But why should one assume
that this last piece of knowledge should be a consequence of
physicalism? To put the point slightly differently, imagine that we
discover that who has mental properties on Earth is in part a
function of the behavior of molecules on Saturn. That would of course
tell us that we are deeply wrong in our assumptions about how the world
works. But it would not tell us that we are deeply wrong about
physicalism. (For further discussion of this point, see Paull and Sider
1992, and Stalnaker 1996.)

Some philosophers (e.g. Davidson 1970) have thought of physicalism as a
conceptual or necessary truth, if it is true at all. But most have
thought of it as contingent, a truth about our world which might have
been otherwise. The statement of physicalism encoded in (1) allows a
way in which this might be so. (1) tells us that physicalism is true at
a world just in case the world in question conforms to certain
conditions. But it leaves it open whether or not the actual world
conforms to those conditions as a matter of fact. Perhaps it
is not true of our world that a physical duplicate of it would
be a psychological duplicate. If so, physicalism would not be true at
our world.

But for some it is puzzling that physicalism is stated using modal
notions (i.e. notions such as possible worlds) and nonetheless is
contingent. To see the problem, notice first that, supervenience
physicalism tells us that the physical truths of the world
entail all the truths; hence

(2) The physical truths entail all the
truths.

Now suppose that S is a statement which specifies the
physical nature of the actual world and S* is a statement
which specifies the total nature of the world. (It might be that
neither S nor S* are expressible in languages we can
understand, but let us set this aside.) If supervenience physicalism is
true, it will then be true that:

(3) S entails S*

On the other hand, (3) is clearly a necessary truth. However, if (3) is
a necessary truth, how can physicalism be contingent? After
all, (3) seems equivalent to physicalism. But if the two are
equivalent, how can one be necessary and the other contingent?

But the response to this problem is straightforward. (3) is
necessary, but it is not equivalent to physicalism. Rather,
(3) follows from physicalism given various contingent
assumptions, in particular the assumptions that S and
S*are the statements we say they are — it is
contingent fact, for example that S* summarizes the total
nature of the world. On other hand, (2) is equivalent to
physicalism but it is not necessary. (It is important to bear
in mind here that not all entailment claims are necessary. Consider
‘my aunt's favorite statement entails my uncle's favorite’
— that statement is contingent even though it is most naturally
thought of as an entailment claim.)

(Cf. Horgan 1983, Lewis 1983.) Imagine a possible world W that
is exactly like our world in respect of the distribution of physical
and mental properties, but for one difference: it contains some pure
experience which does not interact causally with anything else in the
world — epiphenomenal ectoplasm, to give it a name. The
problem this possibility presents for (1) is that, if (1) provides the
correct definition of physicalism, and if physicalism is true at the
actual world, then there is no possible world of the kind we
just described, i.e., W does not exist. The reason is that
W is by assumption a physical duplicate of our world; but
then, if physicalism is true at our world, W should be a
duplicate simpliciter of our world. But W is patently not a
duplicate of our world: it contains some epiphenomenal ectoplasm that
our world lacks. On the other hand, it seems quite wrong to say that
W is an impossibility — at any rate, physicalism should not
entail that it is impossible.

In order to solve the epiphenomenal ectoplasm problem, we need to
adjust (1) so that it does not have the truth of physicalism ruling out
W as a possible world. While there are a number of different
proposals about how to do this, one influential proposal is due to Frank Jackson
(cf. Jackson 1993. For earlier proposals and further discussion, see
Horgan 1983 and Lewis 1983.) He proposes replacing (1) with:

(4) Physicalism is true at a possible world w iff
any world which is a minimal physical duplicate of w is a
duplicate of wsimpliciter

By ‘minimal physical duplicate’, Jackson means a possible
world that is identical in all physical respects to the actual world,
but which does not contain anything else; in particular, it does not
contain any epiphenomenal ectoplasm. Unlike (1), (4) does not have
physicalism ruling out W, and so (4) is on the face of it preferable to (1) as
a statement of physicalism.

A different proposal is due to David Chalmers (1996). He proposes
replacing (1) with:

(5) Physicalism is true at a possible world w iff
any world which is a physical duplicate of w is a
positive duplicate of w

By ‘positive duplicate’, Chalmers means a possible
world that instantiates all the positive properties of the actual
world, where in turn a positive property is defined as one that
if instantiated in a world W, is also instantiated by the
corresponding individual in all worlds that contain W as a proper
part (1996, p. 40). Unlike (1), and like (4), (5) does not have
physicalism ruling out W, and so (5) is on the face of it
preferable to (1) as a statement of physicalism.

(Cf. Hawthorne 2002, Leuenberger 2008) Imagine a possible world
similar to ours with respect to the distribution of mental and
physical properties, except for this difference: the relation between
physical facts and mental facts is weaker than
supervenience—mental facts are entailed by physical facts so
long as there are no facts which block that entailment—blockers,
as they are called. For example, being in an overall physical
condition P will necessitate being in pain so long as you do not also
instantiate some further property B. If you are in both P and B you
are not in pain; but if you are in P and not in B, you will be in
pain.

The problem that this possibility raises for supervenience
definitions of physicalism is as follows. Let us suppose that the
relation obtaining at a world W between the mental and the
physical is one of weak necessity as just defined; that is, suppose
that, at W, the mental is necessitated by the physical but
only if certain blockers are absent. Intuitively it would seem that
physicalism is false at W. On the other hand, if physicalism
is defined in the way suggested by Jackson it would be true. After
all, applied to W, Jackson's definition says that physicalism
is true at W just in case any minimal physical duplicate
of W is a duplicate simpliciter. But that seems to be true
of W as we have imagined it. Conclusion: if blockers are
possible, physicalism is false at W, and yet it should not be
false on Jackson's definition.

There are a number of possible responses to the blockers problem. One
is to resist the intuition that physicalism is false at W in
the circumstance described, even if we adopt Jackson's definition of
physicalism. A different response is to adopt a formulation of
physicalism that is weaker than supervenience physicalism; this
strategy is pursued in Leuenberger 2008. A third response is to say that
what the blockers problem brings out is that there is a
difference between two ways that physicalists have sought to respond
to the epiphenomenal ectoplasm problem; in particular, if one adopts
(5) rather than (4) as one's response to the epiphenomenal ectoplasm
problem, this would have the advantage that it does not also face the
blockers problem. For if the relation of the mental to the physical
that obtains at W is one of weak necessity, then not only is
physicalism false but it is also false that any world which is
a physical duplicate of W is a positive duplicate of W
— at some physical duplicate worlds, for example, there will be
no psychological properties at all. None of these responses are
clearly correct however, and the proper treatment of the blockers
problem (and indeed of the epiphenomenal ectoplasm problem, of which
the blockers problem is a development) is an open question in the
literature (For simplicity, I will continue with (1), rather than with
either (4) or (5); nothing in what follows will turn on this
choice.)

Physicalism is intended as a very general claim
about the nature of the world, but by far the most discussion
of physicalism in the literature has been in the philosophy of mind. The
reason for this is that it is in philosophy of mind that we find the
most plausible and compelling arguments that physicalism is false.
Indeed, as we will see later on, arguments about qualia and
consciousness are usually formulated as arguments for the conclusion
that physicalism is false.

While the issue of physicalism is central to philosophy of mind,
however, it is important also to be aware that supervenience
physicalism is neutral on a good many of the questions that are pursued
in philosophy of mind, and pursued elsewhere for that matter. If you
read over the philosophy of mind literature, you will often find people
debating a number of different issues: whether there are
mental states at all; what sort of thing mental states are; to
what extent mental states are environmentally determined. Given the
multifariousness of mental states, it is quite likely that the correct
position will be some kind of combination of these positions. But this
is a question of further inquiry that is irrelevant to physicalism
itself. So physicalism itself leaves many debates in the philosophy of
mind unanswered.

This point is sometimes expressed by saying that supervenience
physicalism is minimal physicalism (Lewis 1983): it is intended to
capture the minimal or core commitment of physicalism. Physicalists may
differ from one another in many ways, but all of them must at least
hold supervenience physicalism. (Notice that the idea that (1) captures
the minimal commitment of physicalism is a distinct idea from that of a
minimal physical duplicate which Jackson uses in his attempt to capture minimal
physicalism.)

Two issues here require further comment. First, in some discussions
in philosophy of mind, the term ‘physicalism’ is used to
refer to the identity theory, the idea that mental states or properties
are neurological states or properties (Block 1980). In this use of the
term, one can reject physicalism by rejecting the identity theory — so
by that standard a behaviorist or functionalist in philosophy of mind
would not count as a physicalist. Obviously, this is a much more
restricted use of the term than is being employed here.

Second, one might think that supervenience physicalism is
inconsistent with eliminativism, the claim that psychological states do
not exist, for the following reason. Suppose psychological states
supervene on physical states. Doesn't that mean, contrary to
eliminativism, that there must be some psychological states?
The answer to this question is ‘no.’ For consider: the
telephone on my desk has no psychological states whatsoever.
Nevertheless it is still true (though, admittedly, a little odd) to say
that a telephone which is identical to my telephone in all physical
respects will be identical to it in all psychological respects. In the
sense intended, therefore, one thing can be psychologically identical to
another even when neither has any psychological states.

To what extent does supervenience physicalism capture minimal
physicalism, the core commitment of all physicalists? In order to
answer this question it is worth comparing and contrasting
supervenience physicalism with two alternative statements of
physicalism that one finds in the literature: token and type
physicalism.

Token physicalism is the view that every particular thing in the
world is a physical particular. Here is one formulation of this
idea:

Token physicalism:
For every actual particular (object, event or process) x,
there is some physical particular y such that x =
y.

Supervenience physicalism neither implies nor is implied by token
physicalism. To see that token physicalism does not imply supervenience
physicalism, one need only note that the former is consistent with a
version of dualism, namely property dualism. The mere fact that every
particular has a physical property does not rule out the possibility
that some particulars also have non-supervenient mental properties,
i.e. mental properties that are only contingently related to the
physical. But supervenience physicalism does rule out this
possibility. Since token physicalism does not rule out property dualism
but supervenience physicalism does, the first does not imply the
second.

To see that supervenience physicalism does not imply token
physicalism, consider the United States Court of
Appeals for the Seventh Circuit. This might be thought of as a social or
legal object. But then, according to token physicalism, there must be
some physical object for it to be identical with. But there might be no
physical object (in any natural sense of the term) which is identical
to the US Court of Appeals for the Seventh Circuit. On the other hand,
supervenience physicalism imposes no such requirement, and so
supervenience physicalism does not imply token physicalism (For the
classic presentation of this point, see Haugeland 1983).

The point that supervenience physicalism is logically distinct from
token physicalism is an important one. One thing it shows is that
token physicalism (since it is consistent with property dualism) does
not capture minimal physicalism, and so the distinction between token
physicalism and supervenience physicalism is no objection to the
latter. But the difference between the two theses also raises a
different question. Given that token physicalism does not capture the
minimal commitment of physicalism, why has token physicalism been the
subject of such discussion? One reason is that token physicalism
provides one version of the idea that upper level scientific claims
requires physical mechanisms. Supervenience physicalism does not on
its own entail this. But token physicalism is often seen as a way to
ensure this requirement. (For the classic presentation of this point,
see Fodor 1974; see also Papineau 1996. And for a different view
about token physicalism see Latham 2001.)

Having considered token physicalism, we can now turn to type
physicalism. Type physicalism is a generalization and extension of the
identity theory, which we considered above. It holds that that every
property (or at least every property that is or could be instantiated
in the actual world) is identical with some physical property. Here is
a statement of this sort of idea:

Type physicalism:
For every actually instantiated mental property F, there is
some physical property G such that
F=G.

Unlike token physicalism, type physicalism certainly does
entail supervenience physicalism: if every property instantiated in the
actual world is identical with some physical property, then a world
identical to our world in physical respects will of course be identical
to it in all respects.

Nevertheless the reverse entailment does not hold. Supervenience
physicalism, as we have been understanding it, is consistent with the
possibility (if not the actuality) of disembodiment. But type
physicalism as defined here is inconsistent with this possibility, at
least if we focus actually instantiated mental properties. To that
extent, supervenience physicalism does not entail type
physicalism.

Earlier we noted that philosophers such as Davidson have thought
that physicalism is a necessary truth. Even on that assumption,
however, it is still not completely obvious that supervenience
physicalism entails type physicalism. The reason for this has to do
with questions concerning the logical (or Boolean) closure of the set
of physical properties — if P, Q and R are
physical properties, which of the various logical permutations of
P, Q and R are likewise physical properties?
On some assumptions concerning closure and supervenience, supervenience
physicalism (construed as a necessary truth) entails type physicalism;
on other assumptions, it doesn't. But the problem is that the
assumptions themselves are difficult to interpret and evaluate, and so
the issue remains a difficult one. It is not necessary for our purposes
to settle the question concerning closure here. (For further discussion
of these issues see Kim 1993, Bacon 1990, Van Cleve 1990, Stalnaker
1996.)

Before the development of the notion of supervenience,
physicalism was often stated as a reductionist thesis. It will
therefore be useful to contrast the supervenience formulation of
physicalism with various reductionist proposals, and also to consider a
question that has received a lot of attention in the literature, viz.,
whether a physicalist must be a reductionist.

The main problem in assessing whether a physicalist must be a
reductionist is that there are various non-equivalent versions of
reductionism.

One idea is tied to the notion of conceptual or reductive analysis.
When philosophers attempt to provide an analysis of some concept or
notion, they usually try to provide a reductive analysis of the notion
in question, i.e. to analyze it in other terms. Applied to the
philosophy of mind, this notion might be thought of entailing the idea
that every mental concept or predicate is analyzed in terms of a physical
concept or predicate. A formulation of this idea is (6):

(6) Reductionism is true iff for each mental predicate
F, there is a physical predicate G such that a
sentence of the form ‘ x is F iff x is
G’ is analytically true.

While one occasionally finds in the literature the suggestion that
physicalists are committed to (6) in fact, no physicalist since before
Smart (1959) has (unqualifiedly) held anything like (6). Adapting Ryle
(1949), Smart supposed that in addition to physical expressions there
is a class of expressions which are topic-neutral, i.e. expressions
which were neither mental nor physical but when conjoined with any
theory would greatly increase the expressive power of the theory. Smart
suggested that one might analyze mental expressions in topic-neutral
(but not physical) terms, which in effect means that a physicalist
could reject (6). It is fair to say that this move is one of the
central innovations of philosophy of mind, a move to a large extent
endorsed and developed later on by functionalists and cognitive scientists.

A different notion of reduction derives from the attempts of
philosophers of science to explain intertheoretic reduction. The
classic formulation of this notion was given by Ernest Nagel (1961).
Nagel said that one theory was reduced to another if you could
logically derive the first from the second together with what he called
bridge laws, i.e., laws connecting the predicates of the reduced theory
(the theory to be reduced) with the predicates of the reducing theory
(the theory to which one is reducing). Here is a formulation of this
idea, where the theories in question are psychology and
neuroscience:

(7) Reductionism is true iff for each mental predicate
F there is a neurological predicate G such that a
sentence of the form ‘x is F iff x is
G’ expresses a bridge law.

Once again, however, there is no reason at all why physicalists need to
accept that reductionism is true in the sense of (7). Indeed, many
philosophers have argued that there are very strong empirical reasons
to deny that anything like (7) is going to be the case. The reason is
this. Many different neurological processes (whether in our own species
or a different one) could underlie the same psychological process —
indeed, given science fiction, even non-neurological processes might
underlie the same psychological process. But if multiple realizability
— as this sort of idea is called — is true, then (7) seems to be
false. (Fodor 1974, but for recent alternative views, see Kim 1993).

A third notion of reductionism is more metaphysical in focus than
either the conceptual or theoretical ideas reviewed so far. According
to this notion, reductionism means that the properties expressed by the
predicates of (say) a psychological theory are identical to the
properties expressed by the predicates of (say) a neurological theory
— in other words, this version of reductionism is in essence a version
of type physicalism or the identity theory. However, as we have seen,
if physicalists are committed only to supervenience physicalism, they
are not committed to type physicalism. Hence a physicalist need not be
a reductionist in this metaphysical sense.

A final notion of reductionism that needs to be distinguished from
the previous three concerns whether mental statements follow a
priori from non-mental statements. Here is a statement of this
sort of idea,

(8) Reductionism is true iff for each mental predicate
F there is non-mental predicate G such that a
sentence of the form “if x is F then
x is G” is a priori.

What (8) says is that if reductionism is true, a priori
knowledge alone, plus knowledge of the physical truths will allow one
to know the mental truths. This question is in fact a highly vexed one
in contemporary philosophy. However, this question is usually debated
in the context of another, viz., the question of a posteriori
and a priori physicalism. It is to that question, therefore,
to which we will now turn.

We saw earlier that if physicalism is true then (3) is true, where
‘S’ is a sentence that reports the entire physical
nature of the world and ‘S*’ is a sentence that
reports the entire nature of the world:

(3) S entails S*

Another way to say this is to say that if physicalism is true, then the
following conditional is necessarily true:

(9) If S then S*

Indeed, this is a general feature of physicalism: if it is true then
there will always be a necessary truth of the form of (9).

Now, if (9) is necessary the question arises whether it is a
priori, i.e. knowable independent of empirical experience, or
whether it is a posteriori, i.e. knowable but not
independently of empirical experience. Traditionally, every statement
that was necessary was assumed to be a priori. However, since
Kripke's Naming and Necessity (1980),
philosophers have become used to the idea that there are truths which
are both necessary and a posteriori. Accordingly many recent
philosophers have defended a posteriori physicalism: the claim
that statements such as (9) are necessary and a posteriori
(cf. Loar 1997). Moreover, they have used this point to try to disarm
many objections to physicalism, including those concerning qualia and
intentionality that we will consider in a moment. Indeed, as we have
just noted, some philosophers have suggested that the necessary a
posteriori provides the proper interpretation of non-reductive
physicalism.

The appeal to the necessary a posteriori is on the surface
an attractive one, but it is also controversial. One problem arises
from the fact that Kripke's idea that there are necessary and a
posteriori truths can be interpreted in two rather different ways.
On the first interpretation — I will call it the derivation view —
while there are necessary a posteriori truths, these truths
can be derived a priori from truths which are a
posteriori and contingent. On the second interpretation — I will
call it the non-derivation view — there are non-derived
necessary a posteriori truths, i.e. necessary truths which are
not derived from any contingent truths (or any a priori truths
for that matter). The problem is that when one combines the derivation
view with the claim that (9) is necessary and a posteriori,
one encounters a contradiction. If the derivation view is correct, then
there is some contingent and a posteriori statement
S# that logically entails (9). However, if S#
logically entails (9) then (since ‘If C, then if
A then B’ is equivalent to ‘If C
& A, then B’) we can infer that the
following is both necessary and a priori:

(10) If S & S# then
S*.

One the other hand, if physicalism is true, and S summarizes
the total nature of the world it seems reasonable to suppose that
S#was already implicitly included in S. In other
words it seems reasonable to suppose that (10) is simply an expansion of
(9). But if (10) is just an expansion of (9), then if (10) is a
priori, (9) must also be a priori. But that means our
initial assumption is false: (9) is not a necessary a
posteriori truth after all (Jackson 1998).

How might an a posteriori physicalist respond to this
objection? The obvious response is to reject the derivation view of
the necessary a posteriori in favor of the non-derivation
view. But this is just to say that if one wants to defend a
posteriori physicalism, one will have to defend the
non-derivation view of the necessary a posteriori. However,
the non-derivation view is controversial. Indeed, the question of
which interpretation of Kripke's work is the right one, is one of the
most vexed in contemporary analytic philosophy. So it is not something
that we can hope to solve here. (For discussion, see Byrne 1999,
Chalmers 1996, 1999, Jackson 1998, Loar 1997, Lewis 1994, Yablo 1999,
and the papers in Gendler and Hawthorne 2004)

We noted above that while supervenience provides an attractive
answer to the completeness question, it is not as popular now as it
once was. Part of the reason for this are the problems mentioned
in Supervenience Physicalism: Further Issues. But
perhaps the most influential consideration here is what I will
call the sufficiency problem, viz., the apparent fact that
supervenience provides a necessary but not sufficient condition for
the truth of physicalism.

One way to bring out the sufficiency problem focuses on
emergentism, a position on the mind-body problem influential
in the first forty years of the twentieth century (Cf. Kim 1998; see
also Wilson 2005; for the historical background to emergentism, see
MacLaughlin 1992.) Emergentism may itself be understood in several
ways, but in the sense that matters to the sufficiency objection, what
is intended is a position that weaves together elements of both
dualism and physicalism. On the one hand, the emergentist wants to
say that mental facts and physical facts are metaphysically
distinct—just as a standard dualist does. On the other hand,
emergentist wants to agree with the physicalist that mental facts are
necessitated by, and so supervene on, the physical facts. If this
sort of position is coherent, (1) does not articulate a sufficient
condition for physicalism. For if emergentism is true, any physical
duplicate of the actual world is a duplicate simpliciter.
And yet, if emergentism is true, physicalism is false.

A different way to bring out the sufficiency problem focuses on the
idea of a necessary being which is essentially nonphysical
(Cf. Jackson 1998). (Some theists believe that God provides an
example of such a being.) If such a non-physical being exists, it is
natural to suppose that physicalism is false. But if physicalism is
defined according to (1), then physicalism may still be true, for it
remains possible that any minimal physical duplicate of the world is a
duplicate simpliciter. So, again, (1) does not formulate a sufficient
condition for the truth of physicalism.

How to respond to the sufficiency problem? Some philosophers
suppose that the issue is so serious that the only thing to do is to
retreat from supervenience physicalism to type physicalism (e.g. Kim
1998). The major burden on this proposal, however, is that, as we
have seen, type physicalism was given up for a very good reason, e.g.,
multiple realizability.

A different suggestion points out that the problem is only genuine
if the cases that generate it are coherent – and are they? One
reason against supposing so is that both seem to violate Hume's dictum
that there are no necessary connections between distinct existences.
According to emergentism, for example, mental and physical properties
are metaphysically distinct, and yet are necessarily connected. And
if the non-physical necessary being exists at all, it will presumably
be necessarily connected to the physical world and yet distinct from
it. However, Hume's dictum is itself a matter of controversy, so it
is unclear if the cases can be dismissed in this way (see Jackson
1993, Stalnaker 1996, Stoljar 2010, and Wilson 2005, 2010).

In view of the difficulty of responding to the sufficiency problem by
either retreating to type physicalism or rejecting the examples that
generate the problem, a natural option at this point is to agree that
(1) does not articulate a sufficient condition for physicalism but
search for a related proposal that does. One suggestion along these
lines, for example, might be to replace (1) with:

(11) Physicalism is true at a possible world w
iff (a) any world which is a physical duplicate of w is a
duplicate of w simpliciter; and (b) no property instantiated
at w is metaphysically distinct from the physical properties
instantiated at w.

Unlike (1), (11) does not have the consequence that the supervenience
is sufficient for physicalism; hence it does not entail that
physicalism is true if either emergentism is true or essentially
non-physical necessary beings exist. Of course, (11) faces the
further problem of saying what metaphysical distinctness is, and more
generally what the relation is between condition (a) and condition
(b). But it might also be supposed on behalf of (11) that the
proponent of the sufficiency objection must already have answers to
these questions, for otherwise the objection could not itself be
advanced.

We have looked at the sufficiency objection and noted one way in
which the supervenience approach might meet it. But one might think
that the problem goes deeper than has been brought out so far.
According to this line of thought, what is lying behind the
sufficiency objection is the fact that (1) is a modal
definition of physicalism, i.e. one in terms of necessity and
possibility. But, the suggestion continues, modal definitions
inevitably face problems of this sort. If so, the proper response to
the objection is to not to retain the basic shape of (1) and add to it
in various ways, but to offer a quite different proposal, i.e. one
that is a non-modal in character.

What might this alternative be like? I think it is fair to say that
this is a currently open question in the literature. So in this
section, I aim only to review three leading candidates and to note
some issues with each.

The first non-modal definition has been explored in most detail by
Andrew Melnyk (see Melnyk 2003 and the references therein). For
Melnyk, a property F realizes a property G if and
only if (a) G is identical to a second-order property, the
property of having some property that has a certain causal or
theoretical role; and (b) F is the property that plays the
causal or theoretical role in question. We may call this notion
'second-order realization' to distinguish it from a different notion
of realization to be considered in a moment.) This suggests:

(12) Physicalism is true at a possible world w
iff every property instantiated at w is either a
physical property or is second-order realized in a physical property.

Suppose we call physicalism so defined second-order realization
physicalism; what is the relation between it and supervenience
physicalism? Supervenience physicalism does not entail second-order
realization physicalism since the fact that a property F
supervenes on a property G does not entail that F is
a second-order property.

Does second-order realization physicalism entail supervenience
physicalism? The usual assumption is that it does; and indeed it is
natural to suppose that any proposed definition of physicalism would
entail supervenience physicalism even if the reverse is not true
(though see Montero 2013 for an argument to the contrary). However, as
Melnyk himself notes at one point (2003, p. 23), there is an issue
here having to do having to do with the definition of a second-order
property, the property of having some property that has a certain
causal or theoretical role. What are the properties involved in
spelling out these causal or theoretical roles? If physicalism is
true at all, it must be true of these properties as much as any other
properties. But then by second-order realization physicalism, these
properties themselves will be either physical or realized by physical
properties. If the first option is taken, the second-order realization
physicalist will stand revealed as holding a version of identity
physicalism (one level up, as it were), and thus will face the
multiple realization objection. If the second option is taken, the
second-order realization physicalist looks committed to an infinite
regress, since now we have further properties realized by physical
properties and, correlatively, further causal or theoretical roles.
To avoid the regress, the realization physicalism might say that these
properties supervene on physical properties. But now it hard to see
the difference between the realization physicalist and the
supervenient physicalist in the first place.

The second non-modal definition of physicalism has been developed
by Wilson 1999, 2011 and Shoemaker 2007. On this view, a
property F realizes a property G if and only if
(a) G has some set of causal powers or features S;
(b) F has some set of causal powers or features S*;
and (c) S is a sub-set of S*. (We may call this
notion 'sub-set realization' to distinguish it from the different
notion of realization just considered.) This suggests:

(13) Physicalism is true at a possible world w
iff every property instantiated at w is either a
physical property or is sub-set realized in a physical property.

Suppose we call physicalism so defined sub-set realization
physicalism; what is the relation between it and supervenience
physicalism? Supervenience physicalism does not entail sub-set
realization physicalism since the fact that a property G
supervenes on a property F does not entail anything about
their causal powers. For example, it may be that F has no
causal powers at all, while G does; this might be the case if
causation is a macro-phenomenon as some philosophers have held it to
be.

Does sub-set realization physicalism entail supervenience
physicalism? Well, there is a problem here too having to do with
(what is sometimes called) a causal theory of properties,
that is, a theory according to which the causal powers or features
that a property bestows on the things that have it are exhaustive of
the nature of that property. Suppose that a causal theory is false.
Then, in principle, one property might sub-set realize another and yet
be quite different from it in nature. And this in turn suggests that
sub-set realization physicalism does not by itself entail
supervenience physicalism. Of course, one might respond by asserting
that the causal theory is true. But to do that is controversial;
indeed, even those philosophers who hold both a sub-set
model and a causal theory want to separate out these two
commitments (e.g. Shoemaker 2007; see also Wilson
2011). Alternatively, one might respond by denying that physicalism
entails supervenience in the first place, by saying that "lack
of...supervenience is compatible with physicalism" (Wilson 2014, 255,
see also Wilson 2011). But this too is controversial; at any rate, the
sufficiency objection by itself provides no reason for doubting that
supervenience is necessary. Hence the status of the sub-set model
remains controversial.

The third non-modal definition focuses on the idea of grounding,
something that has been extensively discussed recently in the
metaphysics literature (see, e.g., Fine 2001, Schaffer 2009, Rosen
2010, Wilson 2014 and the essays in Correia and Schneider 2012).
Intuitively, a property F is grounded in a
property G just in case F holds in virtue
of G, or the instantiation of G explains the
instantiation of F. This suggests:

(14) Physicalism is true at a possible world w
iff every property instantiated at w is either a
physical property or is grounded in a physical property.

Suppose we call physicalism so defined grounding physicalism;
what is the relation between it and supervenience physicalism?
Supervenience physicalism does not entail grounding physicalism, since
the fact that a property F supervenes on a
property G does not entail that F is grounded
by G.

Does grounding physicalism entail supervenience physicalism? Some
philosophers suppose it does (e.g. Rosen 2010) and so for them
grounding physicalism would entail supervenience physicalism. But
others suppose it does not (e.g. Schaffer 2009) which raises the
question of whether a thesis such as (14) by itself provides an
account of physicalism, or whether some compromise between it and (1)
would have to be reached.

Even if grounding physicalism entails supervenience physicalism,
however, there is the further problem that the notion itself is
controversial. Wilson (2014), for example, points out that grounding
per se is similar to supervenience in that it leaves open many of the
questions philosophers of mind are interested in, viz., whether the
mental exists, whether it is reduced to the physical, and whether it
is causally efficacious. She concludes that grounding “cannot
do the work” that its proponents want it to do (2014, 542). One
might respond that this depends on what work grounding physicalism is
supposed to do. For example, if grounding physicalism, like
supervenience physicalism, is intended only to capture minimal
physicalism, in the sense described above (see
Supervenience Physicalism as Minimal Physicalism)
it may be no objection that it fails to answer these questions.
Whatever is the truth about this, however, there is no doubt that the
precise contours of the grounding relation are yet to be made out.
Hence, the proper assessment of grounding physicalism is at this point
unclear.

Earlier we distinguished two interpretative questions with respect to
physicalism, the completeness question and the condition question. So
far we have been concerned with the completeness question. I turn now
to the condition question, the question of what it is for something (an
object, an event, a process, a property) to be physical.

The condition question has received less attention in the
literature than the questions we have been studying so far. But it is
just as important. Without any understanding of what the physical is,
we can have no serious understanding of what physicalism is. After all,
if we say that, no two possible worlds can be physical
duplicates without being duplicates simpliciter, we don't know what
we've said unless we understand what it would take to be a
physical duplicate, as opposed (say) to a chemical duplicate or a
financial duplicate. (The point here is a quite general one: if Thales
says that everything is water, or Up-to-Date-Thales says everything
supervenes on water, we don't understand what he says unless he says
something about what water is. The physicalist is in the same
position.)

So what is the answer to the condition question? If we concentrate
for simplicity on the notion of a physical property, we can discern two
kinds of answers to this question in the literature. The first ties the
notion of a physical property to a notion of a physical theory, for
this reason we can call it the theory-based conception of a physical
property:

The theory-based conception:
A property is physical iff it either is the sort of property that
physical theory tells us about or else is a property which
metaphysically (or logically) supervenes on the sort of property that
physical theory tells us about.

According to the theory-based conception, for example, if physical
theory tells us about the property of having mass, then having mass is
a physical property. Similarly, if physical theory tells us about the
property of being a rock — or, what is perhaps more likely, if the
property of being a rock supervenes on properties which physical theory
tell us about — then it too is a physical property. (The theory-based
conception bears some relation to the notion of physical1 discussed in
Feigl 1967; more explicit defense is found in Smart 1978, Lewis 1994,
Braddon-Mitchell and Jackson 1996, and Chalmers 1996.)

The second kind of answer ties the notion of a physical property to
the notion of a physical object, for this reason we can call it the
object-based conception of a physical property:

The object-based conception:
A property is physical iff: it either is the sort of property required
by a complete account of the intrinsic nature of paradigmatic physical
objects and their constituents or else is a property which
metaphysically (or logically) supervenes on the sort of property
required by a complete account of the intrinsic nature of paradigmatic
physical objects and their constituents.

According to the object-based conception, for example if rocks, trees,
planets and so on are paradigmatic physical objects, then the property
of being a rock, tree or planet is a physical property. Similarly, if
the property of having mass is required in a complete account of the
intrinsic nature of physical objects and their constituents, then
having mass is a physical property. (The best examples of philosophers
who operate with the object-conception of the physical are Meehl and
Sellars 1956 and Feigl 1967; more recent defense is to be found in
Jackson 1998.)

It is important to note that both conceptions of the physical remain
silent on the question of whether topic-neutral or functional
properties should be treated as physical or not. To borrow a phrase
from Jackson (1998), however, it seems best to treat these properties
as onlooker properties: given any set of physical properties, one might
add onlooker properties without compromising the integrity of the set.
But onlooker properties should not be treated as being physical by
definition.

Do these conceptions characterize the same class of properties?
There are a number of different possibilities here, not of all of
which we can discuss. But one that has received some attention in the
literature is that physical theory only tells us about the
dispositional properties of physical objects, and so does not tell us
about the categorical properties, if any, that they have — a
thesis of this sort has been defended by a number of philosophers,
among them Russell (1927), Armstrong (1968), Blackburn (1992) and
Chalmers (1996). However, if this is correct, it would seem that the
physical properties described by the theory conception are only a
sub-class of the physical properties described by the object
conception. For if physical objects do have categorical properties,
those properties will not count as physical by the standards of the
theory conception. On the other hand, there seems no reason not to
count them as physical in some sense or other. If that is right,
however, then the possibility emerges that the theory- and the
object-conceptions characterize distinct classes of properties.

Along with the concepts of space, time, causality, value, meaning,
truth and existence, the concept of the physical is one of the central
concepts of human thought. So it should not be surprising that any
attempt to come to grips with what a physical property is will be
controversial. The theory and object conceptions are no different:
each has provoked a number of different questions and criticisms. In
this section, I will review some main ones.

One might object that both conceptions are inadequate because they are
circular, i.e., both appeal to the notion of something physical (a
theory or an object) to characterize a physical property. But how can
you legitimately explain the notion of one sort of physical thing by
appealing to another?

However, the response to this is that circularity is only a problem
if the conceptions are interpreted as providing a reductive analysis of
the notion of the physical. But there is no reason why they should be
interpreted as attempting to provide a reductive analysis. After all,
we have many concepts that we understand without knowing how to analyze
(cf. Lewis 1970). So there seems no reason to suppose that either the
theory or object conception is providing anything else but a way of
understanding the notion of the physical.

The point here is an important one in the context of the condition
question. Earlier we said that the condition question was perfectly
legitimate because it is legitimate to ask what the condition of being
physical is that, according to physicalism, everything has. But this
legitimate question should not be interpreted as the demand for a
reductive analysis of the notion of the physical. Consider Thales
again: it is right to ask Thales what he means by ‘water’
— and in so doing demand an understanding of the notion of water
— but it is wrong to demand of him a conceptual analysis of
water.

One might object that any formulation of physicalism which utilizes the
theory-based conception will be either trivial or false. Carl Hempel
(cf. Hempel 1969, see also Crane and Mellor 1990) provided a classic
formulation of this problem: if physicalism is defined via reference to
contemporary physics, then it is false — after all, who thinks that
contemporary physics is complete? — but if physicalism is defined via
reference to a future or ideal physics, then it is trivial — after
all, who can predict what a future physics contains? Perhaps, for
example, it contains even mental items. The conclusion of the dilemma
is that one has no clear concept of a physical property, or at least no concept
that is clear enough to do the job that philosophers of mind want the
physical to play.

One response to this objection is to take its first horn, and insist
that, at least in certain respects contemporary physics really is
complete or else that it is rational to believe that it is (cf. Smart
1978, Lewis 1994 and Melnyk 1997, 2003). But while there is something
right about this, there is also something wrong about it. What is
right about it is that there is a sense in which it is rational to
believe that physics is complete. After all, isn't it rational to
believe that the most current science is true? But even so — and
here is what is wrong about the suggestion — it is still
mistaken to define physicalism with respect to the physics
that happens to be true in this world. The reason is that whether a
physical theory is true or not is a function of the contingent facts;
but whether a property is physical or not is not a function of the
contingent facts. For example, consider medieval impetus
physics. Medieval impetus physics is false (though of course it might
not have been) and thus it is irrational to suppose it
true. Nevertheless, the property of having impetus — the central
property that objects have according to impetus physics — is a
physical property, and a counterfactual world completely described by
impetus physics would be a world in which physicalism is true. But it
is hard to see how any of this could be right if physicalism were
defined by reference to the physics that we have now or by the physics
that happens to be true in our world. (For development of this point,
and for a dilemma that is similar to Hempel's but which casts the
issue in modal rather than temporal terms, see Stoljar 2010.)

A different response to Hempel's dilemma is that what it shows, if
it shows anything, is that a particular proposal about how to define a
physical property — namely, via reference to physics at a particular
stage of its development — is mistaken. But from this one can hardly
conclude that we have no clear understanding of the concept at all. As
we have seen, we have many concepts that we don't know how to analyze.
So the mere fact — if indeed it is a fact — that a certain style of
analysis of the notion of the physical fails does not mean that there
is no notion of the physical at all, still less that we don't
understand the notion.

One might object that, while these remarks are perfectly true, they
nevertheless don't speak to something that is right about Hempel's
dilemma, namely, that for the theory-conception to be complete one
needs to know what type of theory a physical theory is. Perhaps one
might appeal here to the fact that we have a number of paradigms of
what a physical theory is: common sense physical theory, medieval
impetus physics, Cartesian contact mechanics, Newtonian physics, and
modern quantum physics. While it seems unlikely that there is
any one factor that unifies this class of theories, perhaps
there is a cluster of factors — a common or overlapping set of
theoretical constructs, for example, or a shared methodology. If so,
one might maintain that the notion of a physical theory is a
Wittgensteinian family resemblance concept. However, whether this is
enough to answer the question of what kind of theory a physical theory
is remains to be seen.

Hempel's dilemma against the theory-conception is similar to an
objection that one often hears propounded against the object-conception
(cf. Jackson 1998). Imagine the possibility of panpsychism, i.e. the
possibility that all the physical objects of our acquaintance are
conscious beings just as we are. Would physicalism be true in that
situation? It seems intuitively not; however, if physicalism is defined
via reference to the object-conception of a physical property then it
is hard to see why not. After all, according to that conception,
something is a physical property just in case it is required by a
complete account of paradigmatic physical objects. But this makes no
reference to the nature of paradigmatic physical objects, and
so allows the possibility that physicalism is true in the imagined
situation.

One thing to say in response to this objection is that the mere
possibility of panpsychism cannot really be what is at issue here. For
no matter how implausible and outlandish it sounds, panpsychism per se
is not inconsistent with physicalism (cf. Lewis 1983). After all, the
fact that there are some conscious beings is not contrary to
physicalism — why then should the possibility
that everything is a conscious being be contrary to
physicalism? If so, what is at issue in the objection is not
panpsychism so much as the possibility that the paradigms or exemplars
in terms of which one characterizes the notion of the physical might
turn out to be radically different from what we normally assume in a
quite specific sense — they might turn out to be in some
essential or ultimate respect mental.

Once the problem is put like that, however, the panpsychism problem
looks similar to a problem that arises in general whenever one one
tries to understand or define a concept in terms of paradigmatic
objects which fall under it, viz., that these definitions have certain
sort of empirical presuppositions that might turn out to be
false. Suppose one tried to define the concept red in terms of
similarity to paradigmatic red things, such as blood. Pursuing this
strategy commits one to the idea that the belief that blood is red is
a piece of common knowledge shared among all those who are competent
with the term. But that seems wrong — someone who thought that
blood was green would be mistaken about blood but not about red. Now
this problem is a difficult problem, however — and this is the
crucial point for our purposes — the problem is also a quite
general problem; it arises because of the paradigm style of
definition. So to that extent, the concept of the physical does not
seem to be any worse off than the concept of red, the panpsychism
problem notwithstanding. (For discussion of the general strategy see
Lewis 1997)

Of course, one would reject this entire line of thought if one
rejected its starting point, viz., that panpsychism is consistent with
physicalism. Wilson (2006, 78-9), for example, suggests that while
physicalism is consistent with the view that some conscious beings
exist, it is not consistent with the view that some fundamental
conscious beings exist, and it is this last claim that is definitive
of panpsychism. But in fact even that is consistent with physicalism,
though admittedly of an unusual sort. To illustrate, imagine a world
in which the fundamental properties are both mental and physical.
That is certainly a far-fetched scenario but it doesn't seem to be
impossible. Would physicalism be true in such a world? It is hard to
see why not; at least it may be true at that world that any physical
duplicate of it is a duplicate simpliciter. Would panpsychism
likewise be true at such a world? Again, it is hard to see why not,
since the fundamental properties instantiated at such a world are
mental, though of course they are also physical.

One idea that often emerges in the context of Hempel's dilemma and
the panpsychism problem, but deserves separate treatment, is the
so-called Via Negativa (see e.g. Montero and Papineau 2005, Wilson
2006).

The simplest way to introduce the Via Negativa is to interpret it
as a definition of the notion a physical property something like
this: F is a physical property if and only if F is a
non-mental property. But there are many reasons to resist such a
definition. Take vitalism. Vitalism isn't true, but it might have
been true; there is no contradiction in it for example. So imagine a
world in which plants and animals instantiate the key property
associated with vitalism, viz., élan vital. It seems
reasonable to say that in that case plants and animals instantiate a
property that is non-physical, i.e. élan vital is not physical.
And yet one should not say on this account that plants and animals
instantiate a mental property, i.e., élan vital is not mental.
In short, élan vital is neither mental nor physical. But the
Via Negativa as stated cannot accommodate that fact.

One might try to meet this objection by revising the Via Negativa so
that what is intended is only a partial definition along these
lines: F is a physical property only if F is non-mental.
Even so problems remain. As we have seen élan vital causes a
problem because it is neither mental nor physical. But there might be
properties that are both mental and physical. Consider a version of
the identity theory according to which being in pain just is c-fibers
firing. If we suppose that such a theory is true, is the property of
being in pain then mental or physical? Both presumably; but this
could not be true on the Via Negativa construed as a definition of
what a physical property is, even a partial definition. For if a
property is mental and physical, then, given the Via Negativa, it will
be both mental and non-mental which (of course) it can't be! Now
obviously, there are good questions about whether an identity theory
along these lines is or could be true, but regardless of whether it is
true, it should not be ruled out simply because of a proposal about
how to define the words in which it is stated.

Alternatively, one might try to meet the objection by adopting what
Wilson 2006 calls the 'no fundamental mentality' constraint. On this
interpretation, what proponents of the Via Negativa have in mind is
that F is a physical property only if F is not
fundamentally mental, where in turn to be 'not fundamentally mental'
is most naturally understood as entailing that if F is a
fundamental property then it is non-mental. This version of the view
avoids the problem about having c-fibers since presumably that
property is not fundamental. But once again problems remain. Take the
world we considered above at which the fundamental properties are both
mental and physical; in effect, what applies to c-fibers firing (if
the identity theory is true) applies to the fundamental properties
instantiated at this world. As I said, this scenario is far-fetched,
but it doesn’t seem to be impossible, and it is certainly not
impossible simply as a matter of the definition of the words. And yet
it would be impossible for that reason if the 'no fundamental
mentality' version of the Via Negativa were true.

Of course, to raise these problems for the Via Negativa is not to deny
that there is something right about it. For example, when we think of
properties that would falsify physicalism we do often think of
*certain* mental properties, e.g., the distinctive properties of
ectoplasm or ESP. However, this fact—that certain mental
properties would, if instantiated, falsify physicalism—can be
captured without defining the physical in general non-mental. A
better way would be to require of any spelling out the notion of the
physical, either the object-based account or the theory-based account,
that it respect that fact that some ( uninstantiated) mental
properties are non-physical.

In view of the difficulties posed by Hempel's dilemma and related
problems, some philosophers have explored the interesting idea that to
be a physicalist is not to hold some thesis or belief – that is, to
hold something that may be true or false – but is rather to adopt a
kind of attitude or stance. As Alyssa Ney (2008, p. 9, see also Van
Fraassen 2002) develops this “attitudinal” view, for example,
“physicalism is an attitude one takes to form one’s ontology
completely and solely according to what physics says exists”.

Now, as with the via negativa, there is certainly something right
about the attitudinal view. As we will see below, contemporary
physicalists are often methodological naturalists, and methodological
naturalists may well hold the attitude Ney describes. Nevertheless,
there is a major problem for the view, viz., that on the face of it
holding this sort of attitude is neither necessary nor sufficient for
being a physicalist.

To see it is not necessary, consider such ancient philosophers as
Democritus or Lucretius. These philosophers are physicalists, or at
least are usually classified that way, i.e., since they held the
doctrine traditionally called ‘materialism’. But they did
not hold the attitude Ney describes, either implicitly or explicitly,
for physics (at least identified sociologically) did not exist in
their day at all.

In response, one might adjust the attitudinal view so that the
‘physics’ towards which one holds the relevant attitude is
not identified sociologically, but is instead understood as a certain
sort of theory considered in the abstract. But then further problems
arise. First, it is now difficult to see the difference between
holding the relevant attitude and simply believing a thesis. If one
resolves to be guided in one's ontology by the truth of a particular
theory, how is that different from just believing the theory? Second,
if one holds an attitude toward a particular theory, Hempel's dilemma
seems to arise again though in a slightly different form. For which
physical theory is meant? If one means current physics, as in fact
Ney suggests, then one might argue that this is not an attitude that
physicalists should reasonably hold, since current physics is
incomplete; and if one means ideal physics, it is hard to see what the
content or nature of the attitude is.

To see that the attitude is not sufficient, imagine a situation in
which physics postulates properties or objects which are like those
postulated by traditional dualists; as Ney puts it imagine “it is the
year 3000 AD and physicists have been forced to introduce irreducible
mental entities into their theory.” (2008, p. 12). In such a
situation, a person might hold the attitude Ney describes, and yet
intuitively not be a physicalist.

In response, Ney agrees that this is a possibility but points out,
first, it would still be reasonable to criticize the people who hold
the attitude – for example, on the grounds that those who hold a
different attitude might have arrived at correct ontology more
quickly – and, second, that it doesn't follow that the attitude
definitive of physicalism is identical to the attitude definitive of
dualism. (The ideas underlying this second point are (a) if one
adopts the attitudinal view about physicalism then one should in
fairness adopt it about dualism as well; and (b) that from the fact
that two attitudes co-incide in a possible situation it does not
follow that they are identical.) However, while both these
suggestions might be true, it is hard to see them as responding to the
basic point that person who holds the attitude Ney describes in the
imagined situation is not correctly described as a physicalist. In
principle, after all, such a person may be criticized in many ways;
moreover, the fact that holding a particular attitude is not
sufficient for being a physicalist does not entail that doing so is
necessary for being a dualist.

Perhaps because of its connection to the physical sciences,
physicalism is sometimes construed as an entire package of views, which
contains the metaphysical thesis I have isolated for discussion as only
one part. If we want a name for the entire package of views including
the metaphysical claim we might call it the Physicalist World
Picture. I will close our discussion of the interpretation
question by considering the relation between physicalism (the
metaphysical claim) and various other items that at least sometimes
have been thought to be a part of the Physicalist World Picture.

(a) Methodological Naturalism: the idea
that the mode of inquiry typical of the physical sciences will provide
theoretical understanding of the world, to the extent that this sort of
understanding can be achieved. Physicalism is not methodological
naturalism because physicalism is a metaphysical thesis not a
methodological thesis.

(b) Epistemic Optimism: the idea that the mode of
understanding typical of the sciences can be used by us, i.e. by human
beings, to explain the world in total, to provide a final theory of the
world. Physicalism is not epistemic optimism because, since commitment
to physicalism does not commit you to methodological naturalism, it
clearly does not commit you to any optimism about the success of that
method in the long run.

(c) Final Theory: the idea that there is a
final and complete theory of the world, regardless of whether we can
formulate it. One might think it obvious that if physicalism is true,
there is a final theory of the world. However, because of some
unclarity in the notion of a theory, the issues here are not cut and
dried. According to some views, something is a theory only if it is
finitely stateable in a language we can understand. If that is so,
clearly physicalism does not entail the idea of a final theory. On a
looser conception of a theory, however, it is reasonable to say that
physicalism entails that there is a final theory.

(d) Objectivity: the idea that the final and
complete theory of world, if it exists, will not involve any essential
reference to particular points of view or experiences. It is reasonable
to say that physicalism entails objectivity. However, given the
possibilities of non-reductive or a posteriori physicalism
even here the issues are not settled. On those approaches, it seems
possible to have irreducible points of view or experiences supervening
on something physical, which compromises objectivity.

(e) Unity of Science: the idea that all the
branches of sciences developed by us will or should be unified into a
single science, usually (but not always) thought of as physics. This
thesis is clearly a methodological thesis about how science ought to
proceed. As we have seen, however, physicalism is a metaphysical thesis
rather than a methodological thesis about how science ought to proceed.
Hence it is not equivalent to the unity of science thesis.

(f) Explanatory Reductionism: the idea that all
genuine explanations must be couched in the terms of physics, and that
other explanations, while pragmatically useful, can or should be
discarded as knowledge develops. Physicalism is not explanatory
reductionism because, as we saw in our discussion of non-reductive
physicalism, physicalism is consistent with the idea that special
sciences are quite distinct from physics. One might say that the
special sciences are concerned with patterns in the physical that
physicists themselves are not concerned with. For that reason the
subject matter of the special sciences is distinct from the subject
matter of physics.

(g) Generality of Physics: the idea that every
particular event or process which falls under a law of the special
sciences (i.e. sciences other than physics) also falls under a law of
physics. In general, this view presupposes a view about laws and
explanation — for example, it implies or seems to imply that special
sciences have laws. But physicalism does not entail any such
thesis.

(h) Causal Closure of the Physical: the idea that
every event has a physical cause, assuming it has a cause at all.
Strictly speaking, physicalists are not committed to realism about
causation, so they are not committed to causal closure. (Of course,
many physicalists do think that causal closure is true, as we will see
below, but their position does not entail causal closure.)

(i) Empiricism: the idea that all knowledge (with
the possible exception of conceptual knowledge) is ultimately founded
on sensory or perceptual experience. Empiricism can be given a
descriptive or a normative reading. On its descriptive reading, it is
most likely false. Most of the information that normal humans come to
deploy seems to be caused by both experience and inborn
structure and maturation. On the normative reading, the claim is that
justification is, at the end of the day, based on experience. But this
epistemological thesis has nothing to do with physicalism.

(j) Nominalism: the idea that there are no abstract
objects, i.e., entities not located in space and time, such as numbers,
qualities or propositions. If we assume that abstract objects, if they
exist, exist necessarily, i.e., exist in all possible worlds, then
supervenience physicalism is completely silent on the question of
whether abstract objects exist. All supervenience says is that if a
world is a minimal physical duplicate of the actual world, it is a
duplicate simpliciter. But if abstract objects exist then they clearly
exist in both the actual world and any duplicate of the actual world.
What this suggests is that nominalism is a distinct issue from
physicalism (Schiffer 1987, Stoljar 1996).

(k) Atheism: the idea that there is no God as
traditionally conceived. In the 17th and 18th century, physicalism (or
materialism, as it was then known) was widely but not universally viewed as inconsistent
with belief in God (Yolton 1983). Nowadays, this issue is somewhat less
discussed. Nevertheless, as we noted previously, if God is
thought of as essentially non-physical, then Atheism does seem
to be a consequence of physicalism, at least on some interpretations of
the background modal notions.

Having provided an answer to the interpretation question, I now turn to
the truth question: is physicalism (as we have interpreted it so far)
true? I will first discuss three reasons for supposing that physicalism
is not true. Then I will consider the case for physicalism.

The main argument against physicalism is usually thought to concern
the notion of qualia, the felt qualities of experience. The notion of
qualia raises puzzles of its own, puzzles having to do with its
connection to other notions such as consciousness, introspection,
epistemic access, acquaintance, the first-person perspective and so on.
However the idea that we will discuss here is the apparent
contradiction between the existence of qualia and physicalism.

Perhaps the clearest version of this argument is Jackson's knowledge
argument. (There are also a number of other arguments in this area —
for a very good recent discussion, see Chalmers 1996). This argument
asks us to imagine Mary, a famous neuroscientist confined to a black
and white room. Mary is forced to learn about the world via black and
white television and computers. However, despite these hardships Mary
learns (and therefore knows) all that physical theory can teach her.
Now, if physicalism were true, it is plausible to suppose that Mary
knows everything about the world. And yet — and here is Jackson's
point — it seems she does not know everything. For, upon being
released into the world of color, it will become obvious that, inside
her room, she did not know what it is like for both herself and others
to see colors — that is, she did not know about the qualia
instantiated by particular experiences of seeing colors. Following
Jackson (1986), we may summarize the argument as follows:

P1. Mary (before her release) knows everything physical
there is to know about other people.

P2. Mary (before her release) does not know everything there is to
know about other people (because she learns something about them on
being released).

Conclusion. There are truths about other people (and herself) that
escape the physicalist story.

Clearly this conclusion entails that physicalism is false: for if there
are truths which escape the physicalist story how can everything
supervene on the physical. So a physicalist must either reject a
premise or show that the premises don't entail the conclusion.

There are many possible responses to this argument, but here I will
briefly mention only three. The first is the ability
hypothesis due to Lawrence Nemerow (1988) and developed and
defended by David Lewis (1994). The ability hypothesis follows Ryle
(1949) in drawing a sharp distinction between propositional knowledge
or knowledge-that (such as ‘Mary knows that snow is white’)
and knowledge-how (such as ‘Mary knows how to ride a
bike’), and then suggests that all Mary gains is the latter. On
the other hand, P2 would only be true if Mary gained propositional
knowledge.

A second response appeals to the distinction between a
priori and a posteriori physicalism. As we saw above, the
crucial claim of a posteriori physicalism is that (3) — i.e.
the claim that S entails S* — is a
posteriori. Since (3) is a posteriori, you would need
certain experience to know it. But, it is argued, Mary has not had (and
cannot have) the relevant experience. Hence she does not know (3). On
the other hand, the mere fact that Mary has not had (and cannot have)
the experience to know (3) does not remove the possibility that (3) is
true. Hence a posteriori physicalism can avoid the knowledge
argument. (It is an interesting question which premise of the knowledge
argument is being attacked by this response. The answer depends on
whether (3) is physical or not: if (3) is physical, then the response
attacks P1. But if (3) is not physical, the response is that the
argument is invalid.).

A third response is to distinguish between various conceptions of
the physical. We saw above that potentially the class of properties
defined by the theory-conception of the physical was distinct from the
class of properties defined by the object-conception. But that suggests
that the first premise of the argument is open to interpretation in
either of two ways. On the other hand, Jackson's thought experiment
only seems to support the premise if it is interpreted in the one way,
since Mary learns by learning all that physical theory can
teach her. But leaves open the possibility that one might appeal to the
object-conception of the physical to define a version of physicalism
which evades the knowledge argument.

One of the most lively areas of philosophy of mind concerns the issue
of which if any of these responses to the knowledge argument will be
successful. (See the papers in Ludlow, Nagasawa, and Stoljar 2004.)
The ability response raises questions about whether know-how is
genuinely non-propositional (cf. Lycan 1996, Loar 1997 and Stanley and
Williamson 2001), and about whether it gets the facts right to begin
with (Braddon Mitchell and Jackson 1996). As against a
posteriori physicalism, it has been argued both that it rests on
a mistaken approach to the necessary a posteriori (Chalmers
1996, 1999, Jackson 1998), and that the promise of the idea is
chimerical anyway (cf. Stoljar 2000). The third response raises
questions about the distinction between the object and the theory
conception of the physical and associated issues about dispositional
and categorical properties (cf. Chalmers 1996, Lockwood 1992, and
Stoljar 2000, 2001.)

Philosophers of mind often divide the problems of physicalism into two:
first, there are the problems of qualia, typified by the knowledge
argument; second, there are problems of intentionality. The
intentionality of mental states is their aboutness, their capacity to
represent the world as being a certain way. One does not simply think,
one thinks of (or about) Vienna; similarly, one does
not simply believe, one believes that snow is white. Just as
in the case of qualia, some of the puzzles of intentionality derive
from facts internal to the notion, and from the relation of this notion
to the others such as rationality, inference and language. But others
derive from the fact that it seems difficult to square the fact that
mental states have intentionality with physicalism. There are a number
of ways of developing this criticism but much recent work has
concentrated on a certain line of argument that Saul Kripke has found
in the work of Wittgenstein (1982).

Kripke's argument is best approached by first considering what is
often called a dispositional theory of linguistic meaning. According to
the dispositional theory, a word means what it does — for example, the
word ‘red’ means red — because speakers of the word are
disposed to apply to word to red things. Now, for a number of reasons,
this sort of theory has been very popular among physicalists. First,
the concept of a disposition at issue here is clearly a concept that is
compatible with physicalism. After all, the mere fact that vases are
fragile and sugar cubes are soluble (both are classic examples of
dispositional properties) does not cause a problem for physicalism, so
why should the idea that human beings have similar dispositional
properties? Second, it seems possible to develop the dispositional
theory of linguistic meaning so that it might apply also to
intentionality. According to a dispositional theory of intentionality,
a mental concept would mean what it does because thinkers are disposed
to employ the concept in thought in a certain way. So a dispositional
theory seems to hold out the best promise of a theory of intentionality
that is compatible with physicalism.

Kripke's argument is designed to destroy that promise. (In fact,
Kripke's argument is designed to destroy considerably more than this:
the conclusion of his argument is a paradoxical one to the effect that
there can be no such a thing as a word's having a meaning. However, we
will concentrate on the aspects of the argument that bear on
physicalism.) In essence his argument is this. Imagine a situation in
which (a) the dispositional theory is true; (b) the word
‘red’ means red for a speaker S; and yet (c) the speaker
misapplies the word — for example, S is looking at a white thing
through rose-tinted spectacles and calls it red. Now, in that
situation, it would seem that S is disposed to apply ‘red’
to things which are (not merely red but)
either-red-or-white-but-seen-through-rose-tinted-spectacles. But then,
by the theory, the word ‘red’ means (not red but)
either-red-or-white-as-seen-through-rose-tinted-spectacles. But that
contradicts our initial claim (b), that ‘red’ means red. In
other words, the dispositional theory, when combined with a true claim
about the meaning of word, plus a truism about meaning — that people
can misapply meaningful words — leads to a contradiction and is
therefore false.

How might a physicalist respond to Kripke's argument? As with the
knowledge argument, there are many responses but here I will mention
only two. The first response is to insist that Kripke's argument
neglects the distinction between a priori and a
posteriori physicalism. Kripke often does say that according to
the dispositionalist, one should be able to ‘read off’
truths about meaning from truths a physicalist can accept. (For a
proposal like this, see Horwich 2000.) One problem with this
proposal is, as we have seen, that its background account of the
necessary a posteriori is controversial. As we saw, a
posteriori physicalists are committed to what we called the
non-derivation view about necessary a posteriori truths. But the
non-derivation view has come under attack in recent times.

The second response is to defend the dispositional theory against
Kripke's argument. One way to do this is to argue that the
argument only works against a very simple dispositionalism, and that a
more complicated version of such a theory would avoid these problems.
(For a proposal along these lines, see Fodor 1992 and the discussion in
Braddon-Mitchell and Jackson 1996). A different proposal is to argue
that Kripke's argument underestimates the complexity in the notion of a
disposition. The mere fact that in certain circumstances someone would
apply ‘red’ to white things does not mean that they are
disposed to apply red to white things — after all, the mere fact that
in certain circumstances something would burn does not mean that it is
flammable in the ordinary sense. (For a proposal along these lines see
Hohwy 1998, and Heil and Martin 1998)

As with the knowledge argument, the issues surrounding Kripke's
argument are very much wide open. But it is important to note that most
philosophers don't consider the issues of intentionality as seriously
as the issue of qualia when it comes to physicalism. In different
vocabularies, for example, both Block (1995) and Chalmers (1996)
distinguish between the intentional aspects of the mind or
consciousness, and the phenomenal aspects or qualia, and suggest that
it is really the latter that is the central issue. As Chalmers notes
(1996; p. 24), echoing Chomsky's famous distinction, the intentionality
issue is a problem, but the qualia issue is a
mystery.

The final argument I will consider against physicalism is of a more
methodological nature. It is sometimes suggested, not that physicalism
is false, but that the entire ‘project of physicalism’ —
the project in philosophy of mind of debating whether physicalism is
true, and trying to establish or disprove its truth by philosophical
argument — is misguided. This sort of argument has been mounted by a
number of writers, but perhaps its most vocal advocate has been Noam
Chomsky (2000; see also Searle 1992, 1999).

It is easiest to state Chomsky's criticism by beginning with two
points about methodological naturalism. In general it seems rational to
agree with the methodological naturalists that the best hope for a
theoretical understanding of the world is by pursuing the methods which
are typical of the sciences. It would then seem rational as a special
case that our best hope for a theoretical understanding of
consciousness or experience is by pursuing the methods of the sciences
— by pursuing, as we might put it, the naturalistic project with
respect to consciousness. So Chomsky's first point is that it is
rational to pursue the naturalistic project with respect to
consciousness.

Chomsky's second point is that the physicalist project in philosophy
of mind is on the face of it rather different from the naturalistic
project. In the first place, the physicalist project is, as we have
noted, usually thought of a piece of metaphysics. But there is nothing
metaphysical about the naturalistic project, it simply raises questions
about what we can hope to explain. In the second place, the physicalist
project is normally thought of as being amenable to philosophical
argument, whereas it is completely unclear where philosophical argument
would enter the naturalistic project. In short, there doesn't seem
anything particularly ‘philosophical’ about the
naturalistic project — it simply applies the methods of science to
consciousness. But the physicalist project is central to analytic
philosophy.

It is precisely at the place where the physicalist project departs
from the naturalistic project that Chomsky's criticism begins to take
shape. For insofar as it is different from the naturalistic project,
there are a number of ways in which the physicalist project is
questionable. First, it is hard to see what the project might be — it
is true that throughout the history of philosophy and science one
encounters suggestions that one might find out about the world in ways
that are distinct from the ones used in the sciences, but these
suggestions have always been rather obscure. Second, it is hard see how
this sort of project could recommend itself to physicalists
themselves — such a project seems to be a departure
from methodological naturalism but most physicalists endorse
methodological naturalism as a matter of fact. On the other hand, if
the physicalist project does not depart from the naturalistic project,
then the usual ways of talking and thinking about that project are
highly misleading. For example, it is misleading to speak of it as a
piece of metaphysics as opposed to a piece of ordinary science.

In sum, Chomsky's criticism is best understood as a kind of dilemma.
The physicalist project is either identical to the naturalistic project
or it is not. If it is identical, then the language and concepts that
shape the project are potentially extremely misleading; but if it is
not identical, then there are a number of ways in which it is
illegitimate.

How is one to respond to this criticism? In my view, the strongest
answer to Chomsky accepts the first horn of his dilemma and suggests
that what philosophers of mind are really concerned with is the
naturalistic project. Now, of course, what concerns them is not the
details of the project — that would not distinguish them from working
scientists. Rather they are concerned with what the potential limits of
the project are.

This is a theme which has been pursued by
Thomas Nagel (1983) and allied work by Bernard Williams
(1985). According to them, any form of scientific inquiry will at least
be objective, or will result in an objective picture of the world. On
the other hand, we have a number of arguments — the most prominent
being the knowledge argument — which plausibly show that there is no
place for experience or qualia in a world that is described in purely
objective terms. If Nagel and Williams are right that any form of
scientific inquiry will yield a description of the world in objective
terms, the knowledge argument is nothing less than a negative argument
to the effect that the naturalistic project with respect to
consciousness will not succeed.

If what is at issue is the limits of the naturalist project, why is
the debate so often construed as a metaphysical debate rather than a
debate about the limits of inquiry? In answer to this question, we need
to sharply divorce the background metaphysical framework within which
the problems of philosophy of mind find their expression, and the
problems themselves. Physicalism is the background metaphysical
assumption against which the problems of philosophy of mind are posed
and discussed. Given that assumption, the question of the limits of the
naturalistic project just is the question of whether there can
be experience in a world that is totally physical. Nevertheless, when
properly understood, the problems that philosophers of mind are
interested in are not with the framework themselves, and to that extent
are not metaphysical. Thus, the common phrase ‘metaphysics of
mind’ is misleading.

Having considered one side of the truth question, I will now turn to
the other: what reason is there for believing that physicalism is true?

The first thing to say when considering the truth of physicalism is
that we live in an overwhelmingly physicalist or materialist
intellectual culture. The result is that, as things currently stand,
the standards of argumentation required to persuade someone of the
truth of physicalism are much lower than the standards required to
persuade someone of its negation. (The point here is a perfectly
general one: if you already believe or want something to be true, you
are likely to accept fairly low standards of argumentation for its
truth.)

However, while it might be difficult to assess dispassionately the
arguments for or against physicalism, this is still something we should
endeavor to do. Here I will review two arguments that are commonly
thought to establish the truth of physicalism. What unites the
arguments is that each takes something from the physicalist
world-picture which we considered previously and tries to establish the
metaphysical claim that everything supervenes on the physical.

The first argument is (what I will call) The Argument from
Causal Closure. The first premise of this argument is the thesis
of the Causal Closure of the Physical — that is, the thesis that every
event which has a cause has a physical cause. The second premise is
that mental events cause physical events — for example we normally
think that events such as wanting to raise your arm (a mental event) cause
events such as the raising of your arm (a physical event). The third
premise of the argument is a principle of causation that is often
called the exclusion principle (Kim 1993, Yablo 1992, Bennett 2003). The correct
formulation of the exclusion principle is a matter of some controversy
but a formulation that is both simple and plausible is the
following:

Exclusion Principle
If an event e causes event e*, then there is no
event e# such that e# is non-supervenient on
e and e# causes e*.

The conclusion of the argument is the mental events are supervenient on
physical events, or more briefly that physicalism is true. For of
course, if the thesis of Causal Closure is true then behavioral events
have physical causes, and if mental events also cause behavioral
events, then they must supervene on the physical if the exclusion
principle is true.

The Argument from Causal Closure is perhaps the dominant argument
for physicalism in the literature today. But it is somewhat unclear
whether it is successful. One response for the
anti-physicalist is to reject the second premise and to adopt a version
of what is called epiphenomenalism, the view that mental events are
caused by, and yet do not cause, physical events. The argument against
this position is usually epistemological: if pains don't cause pain
behavior how can it be that your telling me that you are in pain gives
me any reason for supposing you are? It might seem that
epiphenomenalists are in trouble here, but as a number of recent
philosophers have argued, the issues here are very far from being
settled (Chalmers 1996, Hyslop 1999). The crucial point is that the
causal theory of evidence is open to serious counterexamples so it is
unclear that it can be used against epiphenomenalism effectively.

A different sort of response is to reject the causal principles on
which the argument is based. As against the exclusion principle, for
example, it is often pointed out that certain events are
overdetermined. The classic example is the firing squad: both the
firing by soldier A and by soldier B caused the prisoner's death but
since these are distinct firings, the exclusion principle is false.
However, while this line of response is suggestive, it is in fact
rather limited. It is true that the case of the firing squad represents
an exception to the exclusion principle — an exception that the
principle must be emended to accommodate. But is difficult to believe
that it represents an exception that can be widespread. A more
searching response is to reject the very idea of causal closure on the
grounds, perhaps, that (as Bertrand Russell (1917) famously argued)
causation plays no role in a mature portrayal of the world. Once again,
however, the promise of this response is more imagined than real. While
it is true that many sciences do not explicitly use the notion of
causation, it is extremely unlikely that they do not imply that various
causal claims are true.

The second argument for physicalism is (what I will call) The
Argument from Methodological Naturalism. The first premise of this
argument is that it is rational to be guided in one's metaphysical
commitments by the methods of natural science. Lying behind this
premise are the arguments of Quine and others that metaphysics should
not be approached in a way that is distinct from the sciences but
should rather be thought of as continuous with it. The second premise
of the argument is that, as a matter of fact, the metaphysical picture
of the world that one is led to by the methods of natural science is
physicalism. The conclusion is that it is rational to believe
physicalism, or, more briefly that physicalism is true.

The Argument from Methodological Naturalism has received somewhat
less attention in the literature than the Argument from Causal Closure.
But it seems just as persuasive — in fact, rather more so. For how
might one respond? One possibility is to reject its first premise. But
this is not something that most people are attracted to (or at least
are attracted to explicitly.)

The other possibility is to reject its second premise. However, once
it is appreciated what physicalism is — and, more important, what it
is not — it is not terribly clear what this would amount to or what
the motivation for it would be. In the first place, our earlier
discussion shows that physicalism is not inconsistent with explanatory
autonomy of the various sciences, so that one should not reject
physicalism merely because one can't see how to reduce those sciences
to others. In the second place, while it is perfectly true that there
are examples of non-physicalist approaches to the world — vitalism in
biology is perhaps the best example — this is beside the point. The
second premise of the Argument from Methodological Naturalism does not
deny that other views are possible, it simply says that physicalism is
the most likely view at the moment. Finally, one might be inclined to
appeal to arguments such as the knowledge argument to show that
physicalism is false, and hence that methodological naturalism could
not show that physicalism is false. However, this suggestion represents
a sort of confusion about the knowledge argument. As we saw above, if
successful the knowledge argument suggests, not simply that physicalism
is false but that any approach to the world that is compatible with
methodological naturalism is false. But if that is so, it is mistaken
to suppose that the knowledge argument gives one any reason to endorse
anti-physicalism if that is supposed to be a position compatible with
methodological naturalism.

That completes our discussion of physicalism. It is sketchy in
certain places, and this suggests that there is much further work to
be done before we arrive at a final assessment of the doctrine of
physicalism and the role that it plays in contemporary thought. In the
future, however, it would seem to me to be a good idea to think at
least about the following:

(a) The contrast between modal and non-modal definitions of
physicalism. While supervenience, and more generally modal,
definitions of physicalism are attractive, they do raise problems of a
difficult nature—such as the blockers problem and the
sufficiency problem. At the time of writing it is unclear how these
difficulties will be resolved.

(b) The relation between, and the adequacy of, the theory-conception
and the object-conception of the physical. It seems clear that our
thinking about the physical is anchored in part in the ordinary idea
of a physical object and in part in the idea of physics. But it seems
equally clear that these two ideas can and do come apart. What are the
consequences of this for the notion of physicalism?

(c) The role of physicalism in philosophy of mind. As we have seen
for both Nagel and Williams questions about physicalism are really
questions about objectivity in disguise. This raises the more general
issue of whether arguments and theses in philosophy of mind (and other
areas of philosophy) that are formulated in terms of physicalism are
necessarily formulated in that way. Perhaps physicalism here is
simply getting in the way of a proper understanding of how these
problems work?

Meehl, P.E. and Sellars, W.S., 1956, ‘The Concept of
Emergence’, in H. Feigl and M. Scriven (eds.), The
Foundations of Science and the Concepts of Psychology and
Psychoanalysis (Minnesota Studies in the Philosophy of Science:
Volume 1), Minneapolis: University of Minnesota Press,
239–252.

Acknowledgments

The author would like to thank Hossein Ameri, Tim Bayne, Rich
Cameron, Brian Garrett, Robert Pasnau, Stewart Saunders, Jessica
Wilson, and particularly David Chalmers for their help in constructing
this entry. In addition, the author and editors would like to thank
two readers, Joshua R. Stern and Greg Stokley, for discovering
numerous typographical errors on an earlier version. Their volunteer
efforts were entirely unsolicited and very much appreciated.